In all honesty, I've never heard of this concept before. I have heard of yin and yang organs or compass directions before, but never in terms of sides of the body. However, if I was to guess, I suppose that your yang side would be your active side (so if you are right handed, that might be you're yang side), while you're yin side would be your weaker, less active side. Why do you ask?

"Seek not to follow in the footsteps of the men of old; seek what they sought."